Linear or branched alkenes having from 2 to 8, preferably from 2 to 6 and in particular from 2 to 4, carbon atoms are also referred to as lower or light olefins. It is known, for example from WO 2009/013174, that light olefins can be prepared from carbon monoxide (CO) and hydrogen (H2) over metal catalysts, for example iron or cobalt catalysts.
The term Fischer-Tropsch synthesis generally refers to the reaction of carbon monoxide with hydrogen in the presence of a heterogeneous catalyst. A mixture comprising essentially carbon monoxide and hydrogen, preferably prepared from natural gas by reaction with water (or oxygen), is also referred to as synthesis gas.CH4+H2OCO+3H2  (1)
Hydrocarbons, in particular alkanes and olefins, are prepared by means of the Fischer-Tropsch synthesis by reaction of the components of synthesis gas over a suitable heterogeneous catalyst, with the product distribution depending greatly on the reaction parameters set. The chemical reaction equation for the synthesis of olefins is shown below.2nH2+nCO→CnH2nnH2O  (2)
In general, additional hydrogen is formed from water and carbon monoxide in the presence of an iron-comprising catalyst, as a result of which an excess of hydrogen over carbon monoxide is frequently present during the process in a Fischer-Tropsch synthesis. Carbon monoxide and hydrogen are in equilibrium with one another, with the greenhouse gas carbon dioxide (CO2) also being formed on the side of hydrogen (see equation (3)). Furthermore, additional carbon dioxide is formed as by-product in the Fischer-Tropsch synthesis.CO+H2OH2+CO2  (3)
WO 2009/013174 A2 describes a process for preparing olefins by reacting carbon monoxide with hydrogen in the presence of an iron-comprising heterogeneous catalyst which comprises essentially a carbonyl iron powder catalyst having spherical primary particles.
WO-A 2009/051353 describes a process for the direct synthesis of light hydrocarbons from natural gas. In the presence of a nickel-aluminum catalyst, synthesis gas having a molar ratio of carbon monoxide to hydrogen of 1:1.5-2.5 is firstly produced from natural gas in a steam reforming process. In the subsequent Fischer-Tropsch process, light saturated hydrocarbons (C2-C4) and, as by-products, methane, carbon dioxide and C2-C4-olefins are formed from carbon monoxide and hydrogen in the presence of an iron-copper-potassium zeolite catalyst. The by-products carbon dioxide and methane are subsequently recirculated to the process in the steam reforming step.
A problem in the Fischer-Tropsch synthesis (for the preparation of olefins) is the undesirable formation (emission) of carbon dioxide which acts as greenhouse gas and has a damaging effect on the environment.